Answer: b.

Explanation:
Everyone is singular — it means “every person.” We would say “on every person’s mind,” so we should also say “on everyone’s mind.”

Bonnie Trenga explains further for Grammar Girl:

“Although I’ll focus on the words everyone and everybody, the same rules apply to the words no one, nobody, anyone, anybody, someone, and somebody. Earlier I stated that grammarians don’t agree on the issue of indefinite pronouns. There are actually two issues concerning this topic: Are the words everyone and everybody singular or plural? And can I use a plural pronoun (such as their) to refer to these words? Grammarians actually agree that the words everyone and everybody are singular. Grammar Girl (that is I!) herself explains the answer in her upcoming book. She says, everyone sounds like a lot of people, but in grammar land, everyone is a singular noun and takes a singular verb. For example:

* Everyone loves Squiggly. (This is right because everyone is singular and paired with a singular verb, loves.)
* Everyone are happy. (This is wrong because it’s pairing the singular noun everyone with a plural verb, are.)
* Everyone hates subject-verb agreement. (This is right because everyone and hates are both singular.)

It’s OK to hate subject-verb agreement, but sometimes you just have to do things you don’t want to do. I promise to pick weeds if you promise to make sure your subjects agree with your verbs.”

Thanks to@biggreenpen for the idea for this question!

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